Method of making magnetic sound reproducers



Jn. 2, P JENSEN n METHOD OF MAKING MAGNETIC SOUND REFRODUCERS Original Filed July 25, 1931 2 SheetsmSheet l Jan. 2, 1934. P. 1.. .JENSEN METHOD OF MAKING MAGNETIC SOUND REPRODUCERS Y original Filed .July 25, 1931 2 sheets-sheet '2 Patented Jan. 2, 1934 UNITED STATES METHOD F MAKING MAGNETIC SOUND REPRODUCERS Peter L. Jensen, Western Springs, Ill., assignor to Jensen Radio Manufacturing Company, .i

Chicago, Ill., a corporation of Nevada Original Application July 25, 1931, Serial No. 553,091. Divided and this application March 31, 1932. Serial No. 602,168

2 Claims.

This invention relates to magnetic sound reproducers, and particularly to a method of making magnetic sound reproducers of the type disclosed and claimed in my copending applica- 5 tion Serial No. 553,091, led July 25, 1931.

The sound reproducers of the copending application above referred to are of the electrodynamic or moving coil type employing a permanent magnet for producing the necessary lines of force in the air gap. An inner pole-piece of sectionalized construction is employed; the sectionalized construction of the "inner pole-piece lending itself to assembly Where the air gap in the outer pole-piece or permanent magnet part is relatively small, and where the distance between the two arms of the outer pole-piece or permanent magnet part is less than the length of the inner pole-piece.

The present application is a division of my copending application above identified, and its object is to provide an improved method of and means for centering the inner pole-piece with respect to the circumferential or outer pole-piece part, and for mounting and holding the inner pole-piecepart in proper centered relation with respect to the other part.

Other objects and advantages of the present invention will be apparent from the following detail description taken in connection with the accompanying drawings, in which I have disclosed the method of my present invention in connection with the magnetic sound reproducer of the parent application above referred to.

In the drawings:

Figure 1 is a section taken longitudinally along the axis of a device, in which the inner polepiece part is positioned concentrically with respect to the opening in the outer pole-piece or permanent magnet part according to the method 40 of my present invention;

Figure 2 is a fragmentary section showing another form of device, in which the inner polepiece part is positioned according to the method of my present invention;

Figure 3 is a perspective view of one part of the inner pole-piece;

Figure 4 is a perspective view of the other part of the inner pole-piece v Figure 5 is a perspective View of the tube for excluding foreign particles from the air-gap;

Figure 6 is a more or less diagrammatic view,

partially in section and partially in elevation, showing the manner of positioning the smaller part of the inner pole-piece concentrically with tending into and centered in the opening 6, I

respect to the annular air-gap between it and the outer pole-piece; and

Figure '7 is a similar View showing how the opening through which the base or larger part of the inner pole-piece is fastened to the adjacent 66 arm of the permanent magnet may be centered similarly With respect to the air-gap forming opening in the opposite arm of the magnet.

The sound reproducer shown comprises a permanent magnet 5 bent to the shape to give a 65 minimum leakage of lines of force, which I find substantially that shown in Figure 1.

The annular air-gap 6 is formed by boring the correct dimensional opening 7 in the arm 8 of the magnet 5, and the correct dimensional airgap is obtained by providing the arm 9 of the magnet 5 with a special pole-piece structure designated generally at l0. The smaller end of this pole-piece structure 10 extends into the opening 7 in the arm 8, forming the annular air-gap 6 so that it will be equi-dimensional at all points.

Due to the extreme small clearance permissible between the voice coil l2 and the walls of the air-gap 6, it is necessary to space the inner polepiece with great precision with respect to the outer or circumferential pole-piece. It is also highly desirable, in order to reduce the reluctance of the magnetic circuit, to make the inner polepiece 10 of large cross section where it is attached to the arm 9 of the permanent magnet 5, and to decrease gradually or by steps the area or cross section of the inner pole-piece as it extends toward and into the opening 7 in the arm 8,

which forms the pole-piece of opposite polarity.

As a permanent magnet is usually relatively heavy and unyielding, it is impossible to insert the base of the inner pole-piece through the opening 6. In order to permit positioning of the base of the inner pole-piece With the smaller end exmake the inner pole-piece of sectionalized construction comprising a smaller part 14 and a base partl 15. The base part 15 has a fiat and relatively large base 16 formed normal to its axis and seating upon the adjacent surface of the arm 9. The outer periphery of this part 15 is cylindrical at 1'7 toward the relatively large base end, and toward the opposite end the periphery of the inner pole-piece part 15 is of conical formation at 18, tapering toward the smaller polepiece part 14. The relatively large cross section of the base part 15 where it is attached to the arm l9 reduces the reluctance of the magnetic circuit, and the relatively large base surface 16 provides a highly satisfactory seat when attached to the adjacent arm of the permanent magnet.

It will be noted that the opening 6 in the arm 8 is beveled or countersunk at 20 from the side away from the inner pole-piece 10. This is advantageous as it produces a concentration of lines of force into the air-gap and at the same time it keeps the leakage between the inner pole-piece and the adjacent arm of the magnet at a minimum. In Figure 1 I have shown the countersinking of the outer end of the opening 7 in the form of a fiat bevel or taper.

In the embodiment of Figure 2 a soft iron insert 21 is secured in an opening 22 in the arm 8 of the permanent magnet, and this insert 21 has the opening 7' which, with the adjacent end.

of the inner pole-piece part 14 projecting into the opening, forms the annular air-gap 6. In this instance the insert 21 is dished inwardly at 23 to the opening 7', which disposes this opening at least partially inwardly of the plane of the arm 8. The outer surface of the insert 21 is provided with. a downwardly dished concavity 24 for the purpose of minimizing leakage between the inner pole-piece and the iron insert, and for concentrating the lines of force into the air-gap 6'. This form of outer pole-piece in conjunction with the arrangement of the inner pole-piece with respect thereto affords the least possible leakage between the inner and outer pole-pieces, except in the air-gap proper 6 where the greatest possible magnetic density is desired. In addition, the soft iron insert 21 possesses less reluctance than the hardened magnet itself, and a desired gain in lines of force in the air-gap is noted.

In the usual types of electrodynamic speakers employing an electromagnet for producing the constant magnetic field, a relatively large top plate or top pole-piece is generally employed. This provides a relatively large seat for the cone housing, and makes the entire construction quite rigid when the speaker is attached to the baffle by means of screws passing through the cone housing. In the present device this seat is relatively small compared with the diameter of the cone and the weight of the complete speaker. For this reason it is desirable to anchor the cone housing 25 to the permanent magnet 5 at a point located at as great a distance as practical from the apex of the cone. This is attained by bendin g the permanent magnet 5 into the shape shown in Figure 1, and fastening the cone housing 25 to the outer arm 8 of the magnet 5 by means of a screw 26 passing through the outer periphery of the cone housing and through the arm 8 at a point close to the highest part of the magnet and near the periphery of the cone housing, and engaging a nut 27 with the opposite end of the screw 26. The inner radial portion 28 of the cone housing has the opening 29 and may be fastened to the arm 8 by screws 30 passing through openings in the radial wall 28 and y, through registering openings in the arm 8, and

` having threaded engagement in nuts 31.

In order to keep the air-gap permanently clean, all entrances thereto are screened and the screening is effected in such a manner and by the use of such materials that the acoustic properties of the speaker are not interfered with. For this purpose a tube 34 is employed. The length of this tube is the same as the distance between the two arms 8 and 9. The tube 34 may be of cardboard or any other non-magnetic material, and its inside diameter is such that it fits snugly or tightly over the relatively large cylindrical portion 17 of the inner pole-piece, which cylindrical portion 17 forms the largest diameter of the inner pole-piece. In this manner the tube 34 is held firmly in place, and fitting snugly between the 8O arms 8 and 9 it prevents the entry of foreign particles into the air-gap from the inner side.

In Figure 1 I have also shown the usual openings 36 in the cone housing, these openings 36 being provided to relieve the air pressure on the back of the cone during the operation of the instrument. These openings 36 are closed by covering the same with cloth as indicated at 37, and this cloth preferably is of a character to permit a comparatively free flow of air or sound to pass through it, but positively stops any particles of any size from entering. Finally, the front of the cone to the air-gap 6 is closed by fastening a light piece of similar cloth as indicated at 38 over the flexible spider 39, which may be of thin sheet metal having openings 40. The fiexible spider 39 is secured marginally to the cone or flexible diaphragm 42 at 43, and centrally the spider 39 is fastened by a screw 44 to the outer part 14 of the inner pole-piece, there being a spacing sleeve 45 between the outer end of the inner pole-piece and the central part of the spider 39, and the screw 44 passes -through an opening in the center of the spider and through the spacing sleeve 45, and is threaded into the outer end of the outer part of the inner pole-piece centrally thereof.

The generally conical diaphragm or cone 42 is attached around its outer periphery at 48, to the outer periphery of the cone housing 25, which is flanged at 49, and rings 50, with an intervening covering of cloth 51 which will screen out foreign particles without affecting the acoustic properties of the device, are applied as shown. The inner truncated end of the cone or conical diaphragm is rigidly secured at 52 to an insulating sleeve 115 53 which carries the movable coil 12 constituting the voice coil of the instrument. This coil 12 is wound upon and preferably firmly cemented to the outer surface of the sleeve 53. The outer periphery of the cone is provided inwardly of the rings 50 with annular folds 54 for permitting the desired yieldability.

In the manufacture of permanent magnets, it is difcult to hold the final shape of the magnet to very close tolerances. The bending of a comparatively heavy bar of steel, the chemical contents of which include 6 per cent of chromium, and the successive heat treatments necessary for obtaining the proper hardening, all tend to make it difficult to retain the exact shape in two other- 139 wise intendedly identical magnets. With this condition in mind, I proceed to locate the inner pole-piece in exact relation to the circumferential pole-piece in the following manner:

The permanent magnet 5 is provided with the 135 relatively large air-gap forming opening '7 in its arm 8, and this opening is cylindrical and of a diameter a few thousandths of an inch larger than the outside diameter of the voice coil l2. The arm 9 of the magnet is provided with an opening 14) 60 for receiving a relatively heavy screw-bolt 61, the shank of which passes 'freely through the opening 60 and is threaded into a correspondingly tapped opening at the axis of the inner pole-piece part 15, and securely fastens this part 15 to the 145 arm 8 is passed in through the opening '1 with the gauge in place in the opening and maintaining the concentric disposition of the drill therein, and the drill is then operated to drill or form a seat 64 in the adjacent end of the inner part 15. The drill is of a size corresponding exactly with the diameter of the pole-piece part 14, and preferably slightly smaller than the inside diameter of the voice coil 12, whereby the seat 64 has a correct spaced relationship with respect to the opening '7 in the arm 8 of the magnet. The pole-piece part i4 is then dropped or inserted into this seat and is thereby positioned exactly concentrically with respect to the opening 7, so that the annular airgap 6 is equi-dimensional at all points, and perfect alignment exists between the inner and outer pole-pieces, and predetermined clearance is provided between the voice coil and the inner and outer pole-pieces. The inner end of the inner pole-piece part 14 may be secured in the recess seat 64 in the adjacent end of the pole-piece part 15 by a set-'screw 65, or a plurality of such setscrews.

The clearance between the shank of the screwbolt 6l and the opening 60 in the arm 9 permits the inner pole-piece assembly to be adjusted bodily and as a unit with respect to the opening '7 in the arm 8 by loosening the screw bolt 6l, shifting the inner pole-piece assembly to the desired position, and then tightening the screw bolt 6l to secure the assembly in adjusted position.

In the form of device illustrated in Figure 2, the bottom part l5' of the inner pole-piece is secured to the arm 9 from above, or from the inner side of said arm instead of from below or from the outer side of said arm, as in Figure l, In this case the opening is tapped, and the screw 6l' passes through the pole-piece part l5' from the front, with its head 66 countersunk inwardly of and below the bottom of the recessed seat 6, and the end of the shank of the screw 61 has threaded engagement in the tapped opening 60 to secure iirmly the part 15 to the arm 9. In this case the inner pole-piece part 14 may be seated and secured in the part 15, and the inner pole-piece part 14' may be centered in the opening 7 as in the embodiment of Figure 1. This form has the same centered arrangement of the parts, but lacks the subsequent adjustability permitted in the previous embodiment by reason of the freedom of the screw bolt 6l in the opening 60.

For the purpose of centering the opening 60 of the embodiment of Figure l, or the opening 65 of the embodiment of Figure 2, with respect to f. the air-gap forming opening 7 in the outer arm 8 of the permanent magnet, the opening 60 (Figure 7) may be formed by a drill 70 provided with a gauge 72 closely tting the opening 7 in the arm 8. The drill is passed in through the opening 7 with the gauge in place in this opening,r and maintaining the concentric disposition of the drill therein, and the drill is then operated to drill or v form the opening 60, which may be threaded or not threaded, as desired. It is understood that thel present invention is not limited to speakers pled to an electric out-put circuit by means or a speaker transformer. In the device illustrated the speaker transformer indicated at 75 is mounted on the permanent magnet where the magnetic potential is substantially zero. The point of socalled zero magneticpotential is the point of magnetic indifference, or that point between the two extremes where the attractive force, after continually diminishing as one proceeds from either pole, ceases altogether. One manner of determining this point is by scattering iron lings upon a sheet of paper and agitating it immediately above the magnet. It is to be understood that where I refer to locating the transformer at the point of substantially zero potential of the magnet, that I intend to include positions located at substantially that point or near it.

The device may be provided with a cup-shaped metal enclosure 80, slotted at 81 for the passage of the arms of the magnet into the same and secured at 82 to tabs struck out from the radial portion 28 of the cone housing. The method of the present application and the type of device in which it is employed may vary within the scope of the appended claims.

I claimt 1. The method of mounting and positioning a two-part member upon one of two spaced and integrally connected arms concentrically with respect to an opening in the other arm which comprises passing a relatively small diameter drilling tool through the opening in one arm with a gauge arranged on said tool and engaging in said opening to center the tool therein, operating said tool to form an opening inthe other arm coaxial with respect to said first opening, introducing one part of the two-part member around the arm with said last opening and attaching same to said arm byv means of said opening, passing a relatively large diameter drilling tool through said rst opening with a gauge arranged on said tool and engaging in said opening to center the tool therein, operating said tool to form a recessed seat in the part of the member attached to the other arm coaxial with said rst opening and inserting the other part of said two-part member through said rst opening into said recessed seat and securing same therein in position with its opposite end projecting concentrically into said rst opening.

2. The method of mounting and positioning a two-part member upon one of two spaced and integrally connected arms concentrically with respect to an opening in the other arm which comprises passing a tool through the opening in one arm and centering the same in said opening, operating said tool to form an opening in the other arm coaxial with respect to said first opening, attaching one part of the two-part member to said last arm by means of said last opening, passing a tool through said rst opening and centering the same therein, operating said tool to form a recessed seat in the part of the member attached to the other arm coaxial with said rst opening and inserting the other part of said two-part member through said first opening into said recessed seat and securing same therein in position with its opposite end projecting concentrically into said rst opening.

PETER L. JENSEN.

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